tokfandomcom-20200215-history
Rodentia
Rodents (from Rodere, "to gnaw") are s of the Rodentia, which are characterized by a single pair of continuously growing in each of the upper and lower s. About 40% of all mammal species are rodents (2,277 species); they are found in vast numbers on all continents except . They are the most diversified mammalian order and live in a variety of terrestrial s, including human-made environments. Species can be , (burrowing), or semiaquatic. Well-known rodents include , s, s, s, s, s, s, s, s, s, s and s. Other animals such as s, s, and s, whose incisors also grow continually, were once included with them, but are now considered to be in a separate order, the . Nonetheless, Rodentia and Lagomorpha are s, sharing a and forming the of . Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and defend themselves. Most eat seeds or other plant material, but some have more varied diets. They tend to be social animals and many species live in societies with complex ways of communicating with each other. Mating among rodents can vary from , to , to . Many have of underdeveloped, young, while others are (relatively well developed) at birth. The rodent record dates back to the on the supercontinent of . Rodents greatly diversified in the , as they spread across continents, sometimes even . Rodents reached both South America and Madagascar from Africa and were the only terrestrial l mammals to reach and colonize Australia. Rodents have been used as food, for clothing, as pets, and as in research. Some species, in particular, the , the , and the , are serious , eating and spoiling food stored by humans and spreading diseases. Accidentally of rodents are often considered to be and have caused the extinction of numerous species, such as island birds, previously isolated from land-based predators. Characteristics is hard , whereas the rear is softer e. The act of chewing wears down the dentine, leaving a sharp, chisel-like edge.}} The distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted . These incisors have thick layers of enamel on the front and little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull. As the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a . Most species have up to 22 teeth with no canines or anterior s. A gap, or , occurs between the incisors and the cheek teeth in most species. This allows rodents to suck in their cheeks or lips to shield their mouth and throat from wood shavings and other inedible material, discarding this waste from the sides of their mouths. Chinchillas and guinea pigs have a high-fiber diet; their molars have no roots and grow continuously like their incisors. In many species, the molars are relatively large, intricately structured, and highly cusped or ridged. Rodent molars are well equipped to grind food into small particles. The jaw musculature is strong. The lower jaw is thrust forward while gnawing and is pulled backwards during chewing. Rodent groups differ in the arrangement of the jaw muscles and associated skull structures, both from other mammals and amongst themselves. The , such as the , have a large deep , making them efficient at biting with the incisors. The , such as the brown rat, have enlarged muscles, making them able to chew powerfully with their molars. The , such as the guinea pig, have larger superficial masseter muscles and smaller deep masseter muscles than rats or squirrels, possibly making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscles may allow them to move the jaw further sideways when chewing. The is a specific morphological feature used for storing food and is evident in particular subgroups of rodents like kangaroo rats, hamsters, chipmunks and gophers which have two bags that may range from the mouth to the front of the shoulders. True mice and rats do not contain this structure but their cheeks are elastic due to a high degree of musculature and innervation in the region. While the largest species, the , can weigh as much as 66 kg (146 lb), most rodents weigh less than 100 g (3.5 oz). The smallest rodent is the , which averages only 4.4 cm (1.7 in) in head and body length, with adult females weighing only 3.75 g (0.132 oz). Rodents have wide-ranging morphologies, but typically have squat bodies and short limbs. The fore limbs usually have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility. The majority of species are , walking on both the palms and soles of their feet, and have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including walking, running, burrowing, climbing, al hopping ( s and ), swimming and even gliding. and s, although not closely related, can both glide from tree to tree using that stretch from the fore to the hind limbs. The is fleet-footed and -like, being and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and sizes. Some tails are , as in the , and the fur on the tails can vary from bushy to completely bald. The tail is sometimes used for communication, as when beavers slap their tails on the water surface or house mice rattle their tails to indicate alarm. Some species have vestigial tails or no tails at all. In some species, the tail is capable of regeneration if a part is broken off. with its long whiskers}} Rodents generally have well-developed senses of , hearing, and vision. Nocturnal species often have enlarged eyes and some are sensitive to light. Many species have long, sensitive or vibrissae for touch or . Some rodents have , which may be lined with fur. These can be turned inside out for cleaning. In many species, the tongue cannot reach past the incisors. Rodents have efficient digestive systems, absorbing nearly 80% of ingested energy. When eating , the food is softened in the stomach and passed to the , where bacteria reduce it to its elements. The rodent then practices , eating its own fecal pellets, so the nutrients can be absorbed by the gut. Rodents therefore often produce a hard and dry fecal pellet. In many species, the penis contains a bone, the ; the testes can be located either abdominally or at the groin. occurs in many rodent species. In some rodents, males are larger than females, while in others the reverse is true. Male-bias sexual dimorphism is typical for s, kangaroo rats, solitary s and s; it likely developed due to and greater male-male combat. Female-bias sexual dimorphism exists among s and . It is not understood why this pattern occurs, but in the case of s, males may have selected larger females due to their greater reproductive success. In some species, such as s, sexual dimorphism can vary from population to population. In s, females are typically larger than males, but male-bias sexual dimorphism occurs in alpine populations, possibly because of the lack of predators and greater competition between males. Phylogenetic tree |1= |1= |2= }} |label2= |2= |label2= |2= |1= '' (pikas) |2='' '' (rabbits & hares) }} |label2='Rodentia' |2= |1= '' (gundis) |2= '' (brush-tailed porcupines) |2= '' (mountain degus) |2= '' (North American porcupines) |2='' '' (guinea pigs) }} }} }} }} |2= |1= '' (mountain beavers) |2= '' (New World flying squirrels) |2='' '' (chipmunks) }} }} |2= |1= '' (beavers) |2= '' (kangaroo rats) |2='' '' (pocket gophers) }} }} |label2= |2= |1= '' (deer mice) |2= '' (true mice) |2='' '' (rats) }} }} |label2= |2= '' (birch mice) |2= '' (jumping mice) |2='' '' (pygmy jerboas) }} }} }} }} }} }} }} }} }} }} References Category:Tree of life